Is breast milk iodine concentration an influential factor in growth- and obesity-related hormones and infants' growth parameters?

Iodine, a key constituent of thyroid hormones, plays an indirect role in prenatal and postnatal growth. This study aimed to investigate whether breast milk iodine concentration (BMIC) is associated with growth- and obesity-related hormones and subsequently the infants' anthropometric measures. In present study conducted in Tehran (Iran), 94 lactating mothers and healthy infants who were exclusively breastfed were included. Concentrations of iodine, insulin-like growth factor-1 (IGF-1), adiponectin (AD) and leptin (LP) were measured in breast milk samples collected during 3- to 5-day postpartum. Anthropometric measures of infants were assessed at 6 months of life, and age- and sex-specific z-score values were calculated using the World Health Organization growth standards. The median (interquartile range) iodine, IGF-1, AD and LP concentrations were 232.5 (157.5-296.0) μg L , 15.7 (11.9-21.1) ng ml , 13.2 (5.1-29.8) mg L and 1.16 (0.86-1.70) ng ml in breast milk, respectively. No significant correlations were found between BMIC and IGF-1, AD and LP concentrations during the first few days postpartum. In adjusted regression model, BMIC was positively associated with weight-for-length z score of infants. In the presence of IGF-1, AD or LP, the coefficients of BMIC for weight-for-length z score of infants were β = .003 (P = .021), β = .002 (P = .028) or β = .003 (P = .013), respectively. No other anthropometric measurements were associated with iodine or growth- and obesity-related hormones in breast milk. Our findings indicate that BMIC is a potential contributor to infants' growth status, independent of IGF-1, AD or LP concentrations in breast milk. The underlying mechanisms remain to be elucidated.